1. Field of Invention
The present invention introduces a process and apparatuses for washing a mass of crystal-mother liquor mixture to thereby produce highly purified crystals. A highly efficient fractional solidification process is obtained by combining the present crystal washing process with a crystallization operation and a crystal melting operation; an efficient purification system is obtained by combining a crystal washer of the present invention with a freezer and a melter.
2. Brief Description of the Prior Art
In recent years, a considerable number of fractional crystallization techniques have been developed. Those techniques are described in details in the book entitled "Fractional Solidification", edited by M. Zief and W. R. Wilcox and published by Marcel Dekker, Inc., New York in 1967. There is a step of separating crystals from mother liquor and washing the crystals in a fractional solidification process, and the product purity depends on how effectively this step has been conducted.
Differential countercurrent contacting of crystals and liquids is a relatively new and promising method of separating and purifying crystals. Column crystallization, a process based on this method, has the advantage over older processes of crystallization that products of high purity can be produced in a single piece of equipment. Column crystallization processes are described in Chapter 11 of the "Fractional Solidification" book described. These processes are outlined in the following.
Column crystallization was conceived by P. M. Arnold and described in U.S. Pat. No. 2,540,977 (1951). The process is conducted in a system that comprises a freezing section, a purification section and a melting section. The crystals and the adhering liquid are conveyed from the freezing section through the purification section to the melting section, where melt liquid is formed. A fraction of this melt liquid is removed as the high melting product. The remaining fraction is returned to the purification section as a free liquid for differential countercurrent contacting with the crystals. The impurities in the crystals, in the adhering liquid and in the retained liquid are transferred to the free liquid and are removed from the column as constituents of the low melting product. Thus, it is seen that column crystallization is analogous to distillation in a packed tower.
Separations based on crystallization rarely achieve product purities indicated by phase equilbria for a variety of reasons. High impurity levels result because mother liquor is often occluded in crystal imperfections and is entrapped in crystal agglomerates. The crystal is further contaminated by the large amount of mother liquor held in the crystal mass by surface tension and capillary forces. Impurities are also adsorbed on the crystal surface by chemisorption. A further source of impurity in the crystals is the major or minor amount of solid solubility. In describing a differential countercurrent contacting of crystals and liquids, the term "free-liquid" is used to refer to the liquid that will readily drain from or pass through the solid phase, the term "adhering-liquid" is used to refer to the liquid which is adsorbed on or held in the crystals by surface tension and by capillary forces and cannot be readily drained from the crystals, and the term "solid phase" is used to refer to the sum of crystals and the adhering liquid. These terms are also used in this specification with the same meanings.
One of the best examples of the successful commercial application of countercurrent column fractional crystallization is the Phillips process that has been described by D. L. McKay in Chapter 16 of the "Fractional Solidification" book described. The process is based on inventions made by P. M. Arnold (U.S. Pat. Nos. 2,540,999 (1951), 2,540,083 (1951), and U.S. Pat. No. Re. 24,038 (1955)), J. Schmidt (U.S. Pat. No. 2,617,274 (1952) and U.S. Pat. No. Re. 23,810 (1954)), J. A. Weedman (U.S. Pat. No. 2,747,001 (1956)), and R. W. Thomas (U.S. Pat. No. 2,854,494 (1958)). In this process, chilled slurry feed, from a scraped-surface chiller, enters at the top of a purification column. The crystals are forced down by means of a piston and impure liquid is removed through a wall filter. Wash liquor, produced by melting purified crystals at the bottom of the column, is transported upwards counter-currently to the crystals. The wash liquor may be pulsed upwards.
Schildknecht column crystallizer, described in Chapter 11 of the "Fractional Solidification" book described, uses a spiral placed in a column defined by two concentric tubes and is rotated to convey the crystals in the desired direction.
J. W. Mullin has described the TNO process on page 250, Vol. 7 of "Encyclopedia of Chemical Technology", edited by Kirk and Othmer and published by Wiley Co. In this process, separation is effected by countercurrent washing coupled with repeated recrystallization facilitated by impacting the crystals during their transport through a vertical column. Impacting is achieved by balls bouncing on sieve plates in the vertical column.
The Brodie Purifier have been used in commercial operations since 1974 and is available through Nofsinger Corp., in Kansas City, Mo. The Brodie Purifier uses several rotating helical ribbon tubular crystallizers and has a recovery section, a refining section, a purifying section and a crystal melting section. Feedstock enters the plant at the feed inlet point, located between the recovery and refining sections. The internal stream is continuously cooled under controlled conditions as it flows through the tubes of the recovery section. The portion of the feed that has been depleted of product component (and contains all the impurities) leaves the plant as residue. Crystals of product material are produced and settle in their own mother liquor. These crystals are mechanically conveyed towards the refining section, countercurrent to the internal liquid stream, by a low speed helical ribbon. As the crystals are transported through the richer mother liquor, they continue to grow in size and also increase in purity. Reflux is recrystallized in the refining section. After passing through the refining section, the crystals settle by gravity in the purifying section. Here, a heater melts the purified crystals; a portion of the melt is extracted as product, and a portion is refluxed and rises countercurrent to the bed of crystals in the purifying section. Cooling is achieved by means of a closed coolant system, flowing countercurrent to the process liquid. Small heat imputs are made to all unjacketed and unscraped surface to prevent uncontrolled crystallization.